Reactive effects refer to:

Reactive effects specifically refer to the changes in behavior that participants may exhibit as a result of being aware that they are part of a study. When individuals know they are being observed or evaluated, this awareness can lead them to modify their responses or behaviors, which may not accurately reflect their natural habits or feelings. This phenomenon is often noted in research settings where social desirability or the pressure to conform to expected norms may guide participants to act differently than they normally would, thus potentially distorting the research outcomes.

The other options touch on different methodological issues that can arise in research, such as limitations of sample size, bias from inadequate controls, and confounding variables. While these are all important concepts in the context of designing and interpreting research studies, they do not align with the definition of reactive effects, which is specifically about behavior alteration due to the awareness of study participation.